Remarkably little is known about the psychobiological determinants of sexual disorders in male diabetic patients. The purpose of this proposals is to assess within a cross-sectional framework the role of organic, psychological and relationship factors and their interaction in the sexual functioning of a well- defined population of male diabetics. Specifically, the central aims of the research are: 1) to provide a comprehensive description of the sexual functioning of a sample of male diabetics living in stable sexual relationships; 2) to evaluate the degree and nature of organic impairment in erectile capacity by means of nocturnal penile tumescence (NPT), bulbocavernosus (BC) reflex response latency and penile vascular assessment methods; 3) to assess systematically psychological and marital factors in relation to the sexual functioning of male diabetics; and, 4) to explore the interaction between the psychological/relationship dimensions and the physiological measures of erectile capacity in the development of erectile impotence. A sample of 120 diabetic men in adequate metabolic control, free from coexisting illnesses or medications which are likely to impair sexual function and between the ages of 21 to 65 will be selected for study. They will be compared to 60 healthy non-diabetic controls who are free from sexual dysfunctions and who meet similar inclusion/exclusion criteria. During the initial phase of the protocol, several sessions will be devoted to screening each man, acquiring informed consent, interviewing the subject and his partner to assess psychosexual behavior, evaluating psychopathology, conducting a medical evaluation and administering a battery of psychological instruments. In the second phase, the subjects will be scheduled for three EEG and NPT recording nights and for assessments of penile vascular function and bulbocavernosus reflex response latency. The relation between sexual functioning, biomedical and psychological variables will be explored by analysis of variance, analysis of covariance, correlation analysis and multiple regression techniques. Knowledge of the relative role of organic and psychological factors in the pathogenesis of diabetic sexual disorders is of critical importance for planning predictive studies and for developing effective evaluation and treatment procedures for diabetic patients. The study will provide a useful model for the assessment of sexual functioning in other organic conditions.